Excited state behavior of trans and cis isomers of stilbene and stiff-stilbene. A TD-DFT study

The first part of the isomerization path on the two lowest excited states of trans and cis isomers of stilbene and stiff stilbene is investigated by means TD-PBE0 calculations in gas phase and in heptane solution. Solvent effect are taken into account by the PCM model. The excited-state optimized structures and the computed absorption and emission frequencies are in good agreement with the available experimental results. In all of the examined compounds a significant part of the isomerization process occurs on the HOMO$\rightarrow$LUMO bright state, whereas the role played by other single-excitation states appears negligible. The relative energy barriers on the isomerization paths are consistent with the experimental excited state lifetimes, suggesting an unifying picture of the isomerization process in stilbene-like molecules.